1. Field of the Invention
The present invention relates to a sheet discharging device, an image forming system, and a sheet discharging method.
2. Description of the Related Art
Conventionally, there has widely been known a sheet processing apparatus that executes various post-processes, including alignment, binding, folding, and bookbinding, to a sheet discharged from an image forming apparatus, the sheet processing apparatus being referred to as a sheet post-processing apparatus because it executes the post-processes. The sheet processing apparatus described above has popularly been used. The sheet post-processing apparatus of this type has been greatly expected to handle variety of sheets in recent years. Especially, a color image forming apparatus frequently prints an image on a coated paper (hereinafter referred to as coat paper) that looks the image wonderful for a catalog or leaflet. The coat paper generally has characteristics of:    1) having high surface smoothness;    2) having strong adhesion force between papers; and    3) having low Clark stiffness.
Therefore, these characteristics might deteriorate a stacking property of the coat paper. Specifically, when sheets that have high surface smoothness or that are easy to be charged such as coat paper are discharged and stacked, the discharged sheet might be adsorbed on a stacking surface of a tray unit or on a stacked sheet, resulting in that the discharged sheet might be buckled, or the discharged sheet might push the stacked paper to deteriorate the stacking property.
In view of this, Japanese Laid-open Patent Publication No. 2011-057313 describes a sheet discharging device having a discharging unit that discharges a sheet having an image formed thereon in a sheet discharging direction; and a tray unit that successively stacks the sheets discharged by the discharging unit, the apparatus further including a blowing mechanism that can repeat an operation of blowing air to the back surface of the sheet, one by one, discharged by the discharging unit and of stopping the blowing operation just before the trailing end of the sheet completely passes through the discharging unit. In this invention, air is sent from an air blower located below the sheet discharging unit in order to form an air layer between the lower surface of the discharged sheet and the sheets that have already been stacked, whereby the stacking property is enhanced.
Japanese Laid-open Patent Publication No. 2001-242769 describes an image forming apparatus having an image forming unit that develops an electrostatic latent image formed on an image carrier with toner to form an image onto a sheet material; a sheet material tray unit that stacks the sheet material having the image formed by the image forming unit; and an air blower that evacuates air in the apparatus to the outside and that sends air to the sheet material stacked on the sheet material tray unit, the apparatus further including a control unit that controls a blowing condition of the air blower according to the state of the sheet material stacked on the sheet material tray unit, in order to stabilize the stacking property of the sheet materials on a discharge tray without causing stain on the sheet material or without causing a stacking trouble that means the sheet materials are adhered to each other on the discharge tray for the sheet materials having the image formed thereon.
However, on the sheet tray unit in the inventions described in Japanese Laid-open Patent Publication No. 2011-057313 and Japanese Laid-open Patent Publication No. 2001-242769, a stacking trouble might be caused by the change in the condition such as a type of a sheet, a size of a sheet, or a thickness of a sheet. Specifically, when air is sent from the air blower even after the discharged sheet completely passes through a nip of a discharging roller, the discharged sheet is blown by airflow, so that the drop position is not fixed. Accordingly, the stacking trouble might be caused. Since the air blower is always opened, foreign matters enter the apparatus, which might damage a mechanism in the apparatus.
Therefore, there is a need to secure a satisfactory stacking property upon discharging a sheet by blowing air, and to prevent foreign matters from entering a blowing port.